1. Field of the Invention
The present invention relates to methods and compositions useful to simultaneously control root diseases caused by Gaeumannomyces graminis, Rhizoctonia, and Pythium species. More particularly, the invention relates to a novel method to identify seed-treatment products for potential to simultaneously control the three kinds of root diseases; microorganism strains having activity against the three kinds of root diseases and methods of obtaining the same; and novel compositions which include a fungicide and biocontrol microorganism, which compositions are useful for field control of the combination of the three kinds of root diseases.
2. Description of the Art
Root diseases caused by Gaeumannomyces graminis, Rhizoctonia, and Pythium cause a significant adverse impact on the production of important crops worldwide. The root disease take-all, caused by Gaeumannomyces graminis var. tritici (Ggt), Rhizoctonia root rot, caused by Rhizoctonia solani and R. oryzae, and Pythium root rot caused by any of several Pythium species, notably, Pythium ultimum and P. irregulare, are important root diseases of cereal crops, e.g., wheat, barley, triticale, and rye, worldwide.
Take-all is probably the most important root disease of wheat and related cereals worldwide. This disease is more severe on wheat and triticale than on rye or barley. Among wheats, it tends to be more severe on durum than common wheats. It causes black lesions on both seminal roots (the first roots to emerge from the seed) and the crown roots (secondary roots from the bases of the mainstem and tillers). Take-all also progresses into the crown (tissue where tillers are physically united to the mainstem) and up the tillers and mainstem of infected plants. Plants tend to die as adult but not yet ripened plants, giving a display of white unfilled heads mixed with normal green heads.
Rhizoctonia root rot occurs throughout the United States Pacific Northwest, in Australia, and South Africa, and potentially throughout the temperate regions of the world wherever cereals are grown, especially if grown with reduced or no-tillage (direct drilling). Rhizoctonia root rot caused by R. solani AG8 begins as brown cankerous lesions on the seminal and crown roots that eventually girdles and then severs the roots. Plants with roots pruned off by this disease remain stunted and eventually die without making heads. The disease tends to affect plants in patches and has given rise to other names, such as bare patch disease, purple patch, crater disease, and barley stunt disorder. Of all cereals, barley is especially susceptible to R. solani AG8. Rhizoctonia oryzae infects the embryos of germinating seeds, preventing germination or limiting the formation of seminal roots to only one or two when healthy seedlings produce five or six seminal roots. These two Rhizoctonia species, together with Rhizoctonia cerealis and possibly other Rhizoctonia species occur as different mixtures, depending on the soil, cropping systems, weed management practices, and possibly other factors not yet identified.
Pythium is possibly the most widespread soilborne pathogen of cereals in the world, occurring in virtually all agricultural soils. Pythium damage to cereals begins as embryo infections and associated poor emergence or stand establishment and continues as destruction of the fine lateral rootlets and root hairs. Plants with Pythium root rot have the appearance of plants without enough fertilizer, because the disease limits the absorptive capacity of the root system through destruction of fine rootlets and root hairs. There are several species of Pythium with ability to attack cereals, either embryos of germinating seeds, root tips and fine rootlets, or all of these delicate and usually juvenile or meristematic tissues.
The pathogens responsible for take-all and Rhizoctonia root rot survive as hyphae or mycelium in the tissues of host plants colonized through their parasitic activities. Pythium species survive in soil as thick-walled oospores or sporangia produced from nutrients robbed from the plant through parasitism. Usually, all three diseases develop simultaneously on the same plants, although one root disease may dominate.
Although Pythium species are ubiquitous in agricultural soils cropped to cereals, damage to cereals caused by Pythium species, e.g., reduction in seedling emergence and plant vigor, is greatest in soils kept wet, especially if the soils are also naturally high in clay content and with pH values below 6.0. Allowing volunteer cereals (plants that develop from seed spilled or dropped by the harvester on the soil surface) to grow in the field after harvest of one crop until only 1 or 2 days before planting the next crop, then spraying with an herbicide such as glyphosate (ROUND UP, Monsanto), controls the weeds but greatly favors Rhizoctonia root rot. As another example, planting wheat directly into the standing stubble of a previous wheat crop with soil kept moist by sprinkler irrigation or leaving the soil covered with straw favors all three root diseases.
Wheat and other cereals with root disease yield poorly and return less on investments to the grower. Plants with these root diseases also compete poorly with weeds, thereby making it necessary to spend more on herbicides to control weeds. Cereals with root diseases also leave fertilizer unused in the soil, including nitrates, which then may move by leaching below the rooting zone and eventually into ground water. Growers throughout the world continue to use some form of tillage for production of cereals, largely because tillage helps control these root diseases. Tillage causes soils to be more vulnerable to soil erosion. It also requires more energy, and leads to greater evaporation of water needed for yield. Some farmers attempting to use no-till burn the stubble in their fields in the belief that this will provide some relief from root diseases. Stubble burning is both environmentally detrimental and socially unacceptable, especially to people in urban areas and cities that object to having to breathe the smoke produced by stubble burning.